In this project we are exploring the cellular mechanisms involved in the differentiation of IgA B cells present in mucosal lymphoid nodules (Peyer's patches, PP). In the first phase of this project we demonstrated that surface IgM (sIgM)-bearing B cells stimulated by the mitogen lipopolysaccharide (LPS) showed a high degree of differentiation into surface IgA(sIgA)-bearing B cells provided they were cultured in the presence of cloned T cells obtained from PP. In contrast, sIgM-bearing B cells stimulated by LPS in the absence of T cells or in presence of cloned T cells obtained from spleen did not develop in sIgA-bearing B cells. PP-derived cloned T cells had thus brought about a class-specific isotype switch, and could be designated IgA-specific switch T cells. Since the effect of the switch T cells obtained from PP was to induce PP B cells to become cells bearing surface IgA but not cells producing IgA for secretion, it was possible that additional T cell influence was necessary for terminal differentiation into IgA plasma cells. Accordingly, in the second phase of this project (accomplished during the past year) we conducted studies in which B cells were cultured sequentially, first with LPS and cloned switch T cells, and then with fresh uncloned T cells or T cell factors derived from the latter. In these studies we found that cells exposed in the first culture to PP cloned T cells (but not spleen cloned T cells) were induced to differentiate into IgA plasma cells in the second culture if the latter contained soluble factors derived from fresh T cells stimulated with Con A or T cells themselves stimulated with staph protein A. In parallel experiments we found that cells exposed in the first culture to spleen cloned T cells were induced to differentiate into IgG plasma cells in the second culture if the latter contained the same kinds of T cell influences as before. These studies show that IgA B cell differentiation is a two-step process in which B cells are first induced to express surface IgA under the influence of switch T cells and they are then induced to differentiate into IgA plasma cells under the influence of helper T cells.